System for assessing chloride concentration and corresponding method and sensor

ABSTRACT

Some embodiments are directed to a system for assessing chloride concentration at one predetermined area of a porous or composite material, such as a reinforced concrete structure, including a sensor embedded in the predetermined area of the material, an analyzer connected to the sensor, and a processing module connected to the analyzer. The sensor includes two facing or coplanar electrodes, an intermediate layer arranged between the electrodes, the intermediate layer being in contact with the material of the predetermined area of the structure and including calcium aluminates. The analyzer is configured to apply an alternate current between the electrodes and output an impedance value or capacitance value of the intermediate layer. The processing module is configured to compute a chloride concentration assessment in the predetermined area of the material based on the impedance value or capacitance value outputted by the analyser.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a national phase filing under 35 C.F.R. § 371 of and claims priority to PCT Patent Application No. PCT/EP2016/066212, filed on Jul. 7, 2016, which claims the priority benefit under 35 U.S.C. § 119 of European Patent Application No. 15306128,8, filed on Jul. 9, 2015, the contents of each of which are hereby incorporated in their entireties by reference.

BACKGROUND

Some embodiments relate generally to the field of material durability and more specifically to the assessment of chloride concentration in porous and composite materials, such as reinforced concrete, pre-stressed concrete or mixed steel-concrete structures. More particularly, some embodiments relate to a system and a method for assessing chloride concentration in a predetermined area of a porous or composite material and to a sensor used in this system and method.

Chloride ingress is one of the major factors of reinforced concrete (RC) deterioration affecting structural serviceability and safety. Chloride ions are accelerators of corrosion processes on the rebar surfaces, decreasing the lifetime of the structures. For other materials, the detection of chlorides is an indicator of the waterproofing against seawater or material durability.

Chloride-induced corrosion begins when the concentration of chloride at the steel bars reaches a threshold value that destroys a thin passive layer of corrosion products (caused by the high alkalinity of concrete at the end of construction), which protects steel bars against corrosion. After corrosion initiation, there is a premature deterioration caused by various mechanisms: loss of reinforcement section, loss of steel-concrete bond, concrete cracking and delamination. After steel corrosion starts, the RC physical and mechanical properties decay at rate that depends on the environmental conditions. This deterioration process generates larger repair and maintenance costs with severe impact on the durability and life-cycle performance.

The measurement of chloride content at the concrete cover could be used to estimate the risk of corrosion initiation, and therefore, to enhance or optimize repair and maintenance costs.

The ingress phenomenon of the chloride ions into the concrete is very complex since it depends on many parameters, notably the concrete composition, its cracking state and the climate to which it is exposed.

SUMMARY

Over the past 30 years, different techniques for chloride measuring have been developed, some of them being destructive and invasive, others being non-destructive. Some of them can be even used in situ. These last ones are desirable techniques for maintenance and prediction of RC structures durability.

The most popular techniques are potentiometric and Volhard methods. They measure free and total chlorides in concrete cores extracted from in service structures. However, these techniques are mostly semi-destructive, time-consuming and costly. Furthermore, their destructive nature leads to additional indirect costs such as traffic delay, traffic management, road closures and lost productivity, which increase costs further. Moreover the destructive nature makes difficult a measure of the evolution at the same place on site or in the same sample in lab.

Non-destructive techniques (NDT) also exist. They imply methods that do not change the environment and the futures usefulness of the material where the measurement is taken. These techniques work for example with external or embedded equipment. The most studied and developed general methods could be classified into three types:

(i) ion selective electrodes (ISE),

(ii) electrical resistivity (ER), and

(iii) optical fiber sensor (OFS).

These three NDT types are reviewed in “Non-destructive methods for measuring chloride ingress into concrete: State-of-the-art and future challenges”, M. Torres-Luque, E. Bastidas-Arteaga, F. Schoefs, M. Sanchez-Silva, J. F. Osma, Construction and Building Material, Volume 68, pp 68-81, 2014.

ISE, ER and OFS have shown some advantages: ISE shows a good chemical stability in aggressive environments, ER is sensitive to chloride presence, and OFS shows better sensitivity to chlorides than the others. However, there are some problems that have not been solved yet. For instance, most of these methods are very sensitive to changes in the conditions inside the concrete structure (e.g., changes in temperature, relative humidity, pH), and some of them require a careful calibration process.

More specifically, ISE is very sensitive to the position of the electrodes and to alkalinity and temperature. In addition, the durability of the reference electrode is not adapted to the lifetime of the concrete structure. ER is very sensitive to the water content of the concrete, the steel bars presence, the carbonation and the presence of electromagnetic fields. Finally, OFS is theoretically adapted to measure low values and it is less impacted by environmental factors but the optical fiber is fragile and needs an additional sheath to be isolated from the concrete that is a corrosive medium.

It may therefore be beneficial to provide a measurement method that is non-destructive and that alleviates at least partially the drawbacks of the related art NDT techniques.

Some embodiments therefore use a new type of sensor embedded in the porous or composite material (for example the reinforced concrete structure), this sensor including a calcium aluminate layer adapted for collecting, detecting and measuring free chloride ions coming from the porous or composite material. The collection of free chloride ions by the above-mentioned layer causes changes in the electrical properties of the layer, notably the impedance and the relative permittivity of the layer. The chloride concentration of the porous or composite material in the proximity of the sensor can therefore be assessed based on the impedance and the relative permittivity changes of the layer of the sensor. This sensor is integrated into a system configured to measure these impedance and relative permittivity changes of the layer and to compute on the basis of these changes a chloride concentration assessment of the porous or composite material in the vicinity of the sensor.

More specifically, some embodiments are directed to a system for assessing chloride concentration at one predetermined area of a porous or composite material, such as a reinforced concrete structure, including:

-   -   a sensor embedded in the predetermined area,     -   an analyzer connected to the sensor, and     -   a processing module connected to the analyzer,

wherein the sensor includes two facing or coplanar electrodes, called electrodes, an intermediate layer arranged between the electrodes, the intermediate layer being in contact with the material of the predetermined area and including calcium aluminates,

wherein the analyzer is configured to apply an alternate current between the electrodes and output an impedance value or capacitance value of the intermediate layer, and

wherein the processing module is configured to compute a chloride concentration assessment in the predetermined area based on the impedance value or capacitance value outputted by the analyzer.

In a first embodiment, the electrodes are facing electrodes and the analyzer is configured to output a capacitance value. For computing the chloride concentration assessment in the predetermined area, the processing module is configured to compute a relative permittivity value of the intermediate layer between the electrodes from the capacitance value outputted by the analyzer and to compute the chloride concentration assessment in the predetermined area based on the computed relative permittivity value.

In this embodiment, the frequency of the alternate current is possibly or preferably included in [100 Hz, 5 MHz].

In a second embodiment, the analyzer is configured to measure an impedance value between the coplanar electrodes, by applying an alternate current between these electrodes and the processing module is configured to compute the chloride concentration assessment in the predetermined area based on the measured impedance value.

In this embodiment, the electrodes are possibly or preferably coplanar electrodes. In addition, the frequency of the alternate current is included in the frequency range [100 Hz, 100 kHz] and preferably in one of the following groups of frequency ranges: [16 kHz, 37.5 kHz]; [52 kHz, 65 kHz]; [81 kHz, 99 kHz].

Some embodiments are directed to a method for assessing chloride concentration in a predetermined area of a porous or composite material, such as a reinforced concrete structure, by using a sensor embedded in the predetermined area, the sensor including two facing or coplanar flat electrodes, an intermediate layer arranged between the two electrodes, the intermediate layer being in contact with the material of the predetermined area and including calcium aluminates, the method including:

-   -   measuring a capacitance value or an impedance value of the         intermediate layer by applying an alternate current between the         electrodes; and     -   computing a chloride concentration assessment in the         predetermined area based on the measured impedance value or         capacitance value.

In a first embodiment, the electrodes are facing electrodes and the measured value is a capacitance value of the intermediate layer between these facing electrodes, and the chloride concentration assessment is computed by:

-   -   computing a relative permittivity value of the intermediate         layer between the electrodes, and     -   computing the chloride concentration assessment in the         predetermined area based on the computed relative permittivity         value.

In this embodiment, the frequency of the alternate current is preferably included in [100 Hz, 5 MHz].

In a second embodiment, the measured value is an impedance value of the intermediate layer between the electrodes, and the chloride concentration assessment is computed based on the measured impedance value.

In this embodiment, the electrodes are possibly or preferably coplanar electrodes. In addition, the frequency of the alternate current is included in the frequency range [100 Hz, 100 kHz] and possibly or preferably in one of the following groups of frequency ranges: [16 kHz, 37.5 kHz]; [52 kHz, 65 kHz]; [81 kHz, 99 kHz].

Finally, some embodiments are directed to a chloride sensor to be embedded in a predetermined area of a porous or composite material, such as a reinforced concrete structure, including:

-   -   a housing,     -   at least two facing or coplanar flat electrodes within the         housing,     -   an intermediate layer arranged between the electrodes within the         housing, the intermediate layer being in contact, via at least         one hole in the housing, with the material of the predetermined         area and including calcium aluminates, and     -   pin connectors connected to the electrodes via conductive lines         and arranged for connecting the electrodes to an external         device.

In a particular embodiment, the sensor includes a plurality of pairs of electrodes offset with respect to one another along an axis of the sensor and connected to a plurality of pin connectors, an intermediate layer being arranged between the electrodes of each pair of electrodes and at least a hole being arranged in the housing at the proximity of each pair of electrodes and opening into the intermediate layer.

In a particular embodiment, the calcium aluminates are selected among CA (═CaO.Al₂O₃), C₃A (=3(CaO).Al₂O₃) and C₁₂A₇ (=12(CaO).7(Al₂O₃)).

In a particular embodiment, the material of the housing is fiber glass or Bakelite or ceramic or Teflon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the following description and drawings, given by way of example and not limiting the scope of protection, and in which:

FIG. 1 is a perspective view of an embodiment of a chloride sensor according to the invention;

FIG. 2 is a partial perspective view of the sensor of FIG. 1;

FIG. 3 is an exploded view of the sensor of FIG. 1;

FIG. 4 is a vertical cross-section view along the axis IV-IV of FIG. 1;

FIG. 5 is an enlarged view of a detail A of FIG. 2;

FIG. 6 is an schematic view of a system according to the invention;

FIG. 7 is a flow chart of the successive steps of first method according to the invention;

FIG. 8 shows curves illustrating, for different frequencies, the relative permittivity of a CA layer versus time when 0.7M NaCl solutions are added to CA layer at regular times;

FIG. 9 shows curves illustrating, for different concentrations of NaCl solutions, the relative permittivity of a CA layer versus time;

FIG. 10 shows curves illustrating the relative permittivity of a CA layer for different concentrations of NaCl solutions and different times;

FIG. 11 shows curves illustrating the relative permittivity of a CA layer versus the chloride concentration;

FIG. 12 is a flow chart of the successive steps of second method according to the invention;

FIGS. 13 and 14 show curves illustrating the impedance magnitude and the phase angle versus time of the CA layer exposed to 0.50% w of Cl⁻/w of total solution between two coplanar electrodes;

FIGS. 15 and 16 show curves illustrating respectively the impedance magnitude |Z| and the phase angle versus time of the CA layer exposed to 0.50% w of Cl⁻/w of total solution between the electrodes of a first couple of facing electrodes;

FIGS. 17 and 18 show curves illustrating respectively the impedance magnitude |Z| and the phase angle versus time of the CA layer exposed to 0.50% w of Cl⁻/w of total solution between the electrodes of a second couple of facing electrodes;

FIG. 19 shows curves illustrating the impedance difference ΔZ between dried CA and CA exposed to different NaCl solutions, measured between two coplanar electrodes, versus frequency;

FIG. 20 shows curves illustrating the impedance difference ΔZ versus frequency in the frequency range [16 kHz; 37.5 kHz] at two different times t1 and t13;

FIG. 21 shows curves illustrating the time average impedance difference ΔZ versus frequency in the frequency range [16 kHz; 37.5 kHz];

FIG. 22 shows curves illustrating the impedance difference ΔZ versus frequency in the frequency range [52 kHz; 65 kHz] at two different times t1 and t13;

FIG. 23 shows curves illustrating the time average impedance difference ΔZ versus frequency in the frequency range [52 kHz; 65 kHz];

FIG. 24 shows curves illustrating the impedance difference ΔZ versus frequency in the frequency range [81 kHz; 99 kHz] at two different times t1 and t13; and

FIG. 25 shows curves illustrating the time average impedance difference ΔZ versus frequency in the frequency range [81 kHz; 99 kHz].

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Some embodiments will be described hereinafter for a concrete structure, such as a reinforced, pre-stressed or mixed steel-concrete. Of course, the invention can be applied to other porous or composite materials.

While example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in details. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but on the contrary, example embodiments are to cover all or most modifications, equivalents, and alternatives falling within the scope of the claims. Like numbers refer to like elements throughout the description of the figures.

Before discussing example embodiments in more details, it is noted that some example embodiments are described as processes or methods depicted as flowcharts. Although the flowcharts describe the operations as sequential processes, many of the operations may be performed in parallel, concurrently or simultaneously. In addition, the order of operations may be re-arranged. The processes may be terminated when their operations are completed, but may also have additional steps not included in the figures. The processes may correspond to methods, functions, procedures, subroutines, subprograms, etc.

Methods discussed below, some of which are illustrated by the flow charts, may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine or computer readable medium such as a storage medium. A processor(s) may perform the necessary tasks. Specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. This invention may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. Similarly, it is to be noticed that the term “coupled” should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression “a device A coupled to a device B” should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of device A and an input of device B which may be a path including other devices or means. Unless otherwise defined, all or most terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Some embodiments are directed to a novel embedded sensor including a specific layer reacting with free chloride ions coming from the concrete structure, this reaction causing modifications in the electrical properties (impedance, conductivity and relative permittivity) of the layer. The amount of chloride that ingress in the sensor can therefore be estimated through the electrical properties' changes. According to some embodiments, the specific layer includes calcium aluminate.

FIGS. 1 to 5 illustrate an embodiment of the sensor according to the invention.

The Chloride sensor 1 includes a housing 10 made of three parts 10 a, 10 b and 10 c. Since the sensor is deemed to be imbedded in the concrete, it should be able to face the environment inside the concrete (temperature, humidity, residual inner forces of the concrete). Strong materials that can withstand the environmental conditions are required for the housing. In a possible or preferred embodiment, the housing 10 has a matrix of fiber glass that, in general, shows good physical and chemical properties. Other materials like Bakelite or Teflon can be used.

More specifically, the housing 10 includes a lower part 10 a, an intermediate part 10 b and an upper part 10 c. The lower and upper parts 10 a and 10 c are printed circuit boards (PCBs). Conductive electrodes 11, in copper or gold material, are printed on the lower surface of the upper part 10 c and on the upper surface of the lower part 10 a. Holes 12 are made in the intermediate part 10 b and are filled with a powder of calcium aluminate forming a calcium aluminate layer 13. In this embodiment, the shape of the electrodes 11 and the holes 12 is rectangular.

The electrodes 11 and the holes 12 are positioned relative to each other such that, when the three parts are assembled together, each electrode 11 of the upper part 10 c is facing an electrode 11 of the lower part 10 b, the calcium aluminate layer of a hole 12 being placed between the two electrodes.

In the embodiment illustrated by the FIGS. 1 to 5, the sensor includes eight pairs of facing electrodes 11 and four holes filled with a calcium aluminate layer. The eight pairs of facing electrodes are distributed into fours rows of two pairs of facing electrodes and two columns of four pairs of facing electrodes, one hole 12 (forming a chamber) filled with a calcium aluminate layer being associated to each row of pairs of facing electrodes.

In these figures, two pairs of facing electrodes 11 are associated to the same hole 12 (or chamber) such that a same calcium aluminate layer 13, so-called intermediate layer, is present between the electrodes of these two pairs of electrodes.

Holes 14 are made in the upper part 10 c and/or the lower part 10 a such that, when the sensor is embedded in the concrete structure, the intermediate layer 13 is in contact with the concrete via the holes 14. In the illustrated embodiment, one hole 14 in the upper part 10 c and one hole 14 in the lower part 10 a are made for each row of facing electrodes and open into one hole 12 of the intermediate part. In each part 10 a or 10 c, the holes 14 are offset horizontally with respect to one another in order to be in contact staggered areas of the concrete. Each hole 14 is centred between the coplanar electrodes 11.

Additionally, each electrode 11 of the upper part and the lower part is connected to a pin connector 15 via a conductive line 16. These pin connectors are deemed to connect the electrodes 11 to external devices.

As mentioned above, each hole 12 is filled a powder including calcium aluminates. The powder is for example of a powder including monocalcium aluminates CA (═CaO.Al₂O₃) or tricalcium aluminates C₃A (=3(CaO).Al₂O₃) or C₁₂A₇ or a powder including a mix of the calcium aluminates.

FIG. 5 is a view illustrating the size of different elements of the sensor that will be tested later in the present description. The size of the rectangular hole 12 in the intermediate part is c*d and the size of the rectangular electrodes is a*b. Only a portion a′*b of the electrode 11 a is exposed to the intermediate layer present in the hole 12. The hole 14 is circular and its diameter is 6 Angströms (10⁻¹⁰ m). This diameter is greater than the diameter of a water molecule (few Angströms), the diameter of chloride ions (few Angströms) and the diameter of the concrete's pores (about 1000 Angströms). Thus water molecules and chloride ions can go inside the sensor or reach the intermediate layer via the hole 14.

As can be seen on FIG. 6, this sensor 1 is intended to be imbedded into a reinforced concrete structure RC including a steel bar SB. The sensor 1 is imbedded vertically in the RC structure, perpendicularly to the external wall of the RC structure, in order to measure the chloride concentrations at different depths in the RC structure. In a variant, it can be placed horizontally to measure chloride content in a specific depth.

These measures are carried out by connecting the sensor 1 to an analyzer 2 via connexion lines 3. The analyzer 2 is connected to a processing module 4.

In a first embodiment of the invention, the chloride concentration assessment is computed based on the capacitance of the intermediate layer between two facing electrodes. In this embodiment, the analyzer 2 is a capacitance analyzer.

In a second embodiment of the invention, the chloride concentration assessment is computed based on the impedance value of the intermediate layer between two facing or coplanar electrodes. In this second embodiment, the analyzer 2 is an impedance analyzer.

These two embodiments will be described in more detail hereinafter.

First Embodiment

In this embodiment, the method for assessing the chloride concentration using the above described sensor is detailed in the flow chart of FIG. 7.

In a first step, S1, the capacitance value C between each pair of facing electrodes 11 of the sensor 1 is measured. The capacitance value is measured by applying an alternate current between the electrodes. This capacitance value is measured by the analyzer 2.

In a second step, S2, a relative permittivity value ε_(r) is computed from the capacitance value C with the following equation:

$\begin{matrix} {ɛ_{r} = {\frac{1}{ɛ_{0}} \cdot C \cdot \frac{d}{S}}} & (1) \end{matrix}$

where:

-   -   ε₀ is the vacuum permittivity (ε₀=8.85×10⁻¹² F/m);     -   d is the distance between the two electrodes, and     -   S is the area of the electrodes exposed to calcium aluminate         layer (S=a′*b).

This relative permittivity value ε_(r) is computed by the analyzer 2 and/or the processing module 4.

In a third step, S3, the chloride concentration assessment is computed by the processing module 4 based on the relative permittivity value ε_(r).

In this embodiment, the analyzer 2 is for example the analyzer Agilent 4294A coupled to Dielectric text fixture 16451B. For this specific device, the capacitance can be directly measured, applying an alternate current at frequency range of 100 Hz-5 MHz, with maximum voltage of 0.5V.

This method has been experimented by using a sensor as illustrated by FIGS. 1 to 5. The area S of the electrodes exposed to the intermediate layer 13 is 1.13×10⁻³ m². The experiments are realized at a temperature of 19° C.±1° C. The intermediate layer 13 is done by putting 6.6 g of Monocalcium aluminate powder (CA) in each hole (chamber) 12 and by tamping with a rammer during 120 seconds until it reaches a thickness between 1.84×10⁻³ m and 2.27×10⁻³ m.

Water deionized (0M) and three NaCl solutions with three different NaCl concentrations were used to test the dielectric behaviour of the monocalcium aluminate layer (CA layer): 0.5M, 0.7M and 1.0M.

Table 1 lists the name and characteristics of each test.

TABLE 1 NaCl Concentration Sample NaCl Concentration (% w of Cl⁻/w of name (M—Molar) total solution) CA 0 (Dried) 0 CAH 0 (Hydrated) 0 CACl0.5 0.5 0.0257 CACl0.7 0.7 0.0357 CACl1 1.0 0.0486

In addition, measurements were performed 1 minute after 1 ml (millilitre) of NaCl solution is added to CA and every 10 minutes for 1 hour to determine time-dependency. Each experiment was performed by triplicate. The NaCl solutions were introduced in the sensor by the holes 14.

FIG. 8 shows diagrams representing the computed relative permittivity versus Time of a CA layer when 1 ml of 0.7M NaCl solution is added every 10 minutes and for different frequencies of alternate current. At the beginning, the CA layer is dried.

All or most of these diagrams show the same tendency. In the first 10 minutes of each experiment, the relative permittivity reaches a steady value and remains approximately at this value until the end of the measurement period. These diagrams suggest that relative permittivity does depend on neither the time nor the frequency in this frequency range.

FIG. 9 shows the change in relative permittivity as chloride solution concentration is increased. The effect of chloride solutions is to increase the measured relative permittivity. It means that capacitance between the electrodes arises due to the ingress of Cl and Na ions that caused an ionic polarization inside the material.

FIGS. 10 and 11 show that relative permittivity ε_(r) of the CA layer is proportional to its chloride concentration through the following relation:

ε_(r)=2.438+1.391X  (1)

where: ε_(r) is the relative permittivity and X is the molar chloride concentration.

More specifically, FIG. 10 shows the effect of chloride content on the measured relative permittivity for CA dried and for CA exposed to 0M, 0.5M, 0.7M and 1M NaCl solutions and FIG. 11 shows the correlation between chloride concentration and relative permittivity.

It means that ionic polarization of NaCl and molecular polarization of H₂O lead to higher values of the dielectric constant allowing the increased of stored charge in the CA. Also, ionic penetration into the material causes that electric resistivity decreases, and of course, conductivity increases.

Second Embodiment

In this embodiment, the method for assessing the chloride concentration using the above described sensor is detailed in the flow chart of FIG. 12.

In a first step, S′1, an impedance value between each pair of facing or coplanar electrodes 11 of the sensor 1 is measured. The impedance value is measured by applying an alternate current between the two electrodes. This impedance value is measured by the analyzer 2.

In a second step, S′2, the chloride concentration assessment is computed by the processing module 4 based on the measured impedance value.

In this embodiment, the analyzer 2 is for example the analyzer Agilent 4294A coupled to Kelvin clip 16089A. This instrument works in the frequency range of 100 Hz-100 kHz, at a voltage of 0.5V. This reduction on the frequency is possible due to the steady state that CA showed in FIG. 8.

This method has been experimented by using the same sensor as for the first embodiment.

During experiments, impedance values between facing electrodes and coplanar electrodes of two adjacent pairs of facing electrodes 11 in contact with the CA layer in a same hole 12 were measured, that means 4 measurements:

-   -   1 measurement between the electrodes of the first pair of facing         electrodes 11;     -   1 measurement between the electrodes of the second pair of         facing electrodes 11;     -   1 measurement between the electrodes of the first pair of         coplanar electrodes 11; and     -   1 measurement between the electrodes of the second pair of         coplanar electrodes 11.

For both couples of measurements, impedance shows an inversely behaviour against chloride concentration, that is, the impedance value decreases as chloride concentration increases.

Table 2 lists the experiments for the calibration process, and Table 3 shows the randomization of those experiments. The solutions concentration varies between 0% and 6% w of Cl⁻/w of total solution, and one of the sensors was made empty (NCA) for demonstrating the design suitability. Solutions were directly applied to the holes 14 using rubber tubes and syringes, and two parallel capacitors (between facing electrodes) and only one coplanar capacitor (between coplanar electrodes) were measured in each chamber (hole 12 filled with the CA layer) due to the time of measurement (20 sec aprox.). The main objectives of this design are to determine linearity, time response and sensitivity. Each solution shown in Table 3 was made 4 times. In total, we used 22 devices, 4 repetitions, and 13 measurements over time. Additionally, at the beginning of the experiments we took the impedance of the dried CA.

TABLE 2 Design of Experiment for calibration t(min) 2 4 6 8 10 15 20 25 30 40 50 60 90 [Cl−](%) t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 t13 NCA C0 0.00 C1 C1t1 C1t2 C1t3 C1t4 C1t5 C1t6 C1t7 C1t8 C1t9 C1t10 C1t11 C1t12 C1t13 0.01 C2 C2t1 C2t2 C2t3 C2t4 C2t5 C2t6 C2t7 C2t8 C2t9 C2t10 C2t11 C2t12 C2t13 0.02 C3 C3t1 C3t2 C3t3 C3t4 C3t5 C3t6 C3t7 C3t8 C3t9 C3t10 C3t11 C3t12 C3t13 0.03 C4 C4t1 C4t2 C4t3 C4t4 C4t5 C4t6 C4t7 C4t8 C4t9 C4t10 C4t11 C4t12 C4t13 0.04 C5 C5t1 C5t2 C5t3 C5t4 C5t5 C5t6 C5t7 C5t8 C5t9 C5t10 C5t11 C5t12 C5t13 0.05 C6 C6t1 C6t2 C6t3 C6t4 C6t5 C6t6 C6t7 C6t8 C6t9 C6t10 C6t11 C6t12 C6t13 0.06 C7 C7t1 C7t2 C7t3 C7t4 C7t5 C7t6 C7t7 C7t8 C7t9 C7t10 C7t11 C7t12 C7t13 0.07 C8 C8t1 C8t2 C8t3 C8t4 C8t5 C8t6 C8t7 C8t8 C8t9 C8t10 C8t11 C8t12 C8t13 0.08 C9 C9t1 C9t2 C9t3 C9t4 C9t5 C9t6 C9t7 C9t8 C9t9 C9t10 C9t11 C9t12 C9t13 0.09 C10 C10t1 C10t2 C10t3 C10t4 C10t5 C10t6 C10t7 C10t8 C10t9 C10t10 C10t11 C10t12 C10t13 0.10 C11 C11t1 C11t2 C11t3 C11t4 C11t5 C11t6 C11t7 C11t8 C11t9 C11t10 C11t11 C11t12 C11t13 0.20 C12 C12t1 C12t2 C12t3 C12t4 C12t5 C12t6 C12t7 C12t8 C12t9 C12t10 C12t11 C12t12 C12t13 0.30 C13 C13t1 C13t2 C13t3 C13t4 C13t5 C13t6 C13t7 C13t8 C13t9 C13t10 C13t11 C13t12 C13t13 0.40 C14 C14t1 C14t2 C14t3 C14t4 C14t5 C14t6 C14t7 C14t8 C14t9 C14t10 C14t11 C14t12 C14t13 0.50 C15 C15t1 C15t2 C15t3 C15t4 C15t5 C15t6 C15t7 C15t8 C15t9 C15t10 C15t11 C15t12 C15t13 1.00 C16 C16t1 C16t2 C16t3 C16t4 C16t5 C16t6 C16t7 C16t8 C16t9 C16t10 C16t11 C16t12 C16t13 1.50 C17 C17t1 C17t2 C17t3 C17t4 C17t5 C17t6 C17t7 C17t8 C17t9 C17t10 C17t11 C17t12 C17t13 2.00 C18 C18t1 C18t2 C18t3 C18t4 C18t5 C18t6 C18t7 C18t8 C18t9 C18t10 C18t11 C18t12 C18t13 2.50 C19 C19t1 C19t2 C19t3 C19t4 C19t5 C19t6 C19t7 C19t8 C19t9 C19t10 C19t11 C19t12 C19t13 3.00 C20 C20t1 C20t2 C20t3 C20t4 C20t5 C20t6 C20t7 C20t8 C20t9 C20t10 C20t11 C20t12 C20t13 6.00 C21 C21t1 C21t2 C21t3 C21t4 C21t5 C21t6 C21t7 C21t8 C21t9 C21t10 C21t11 C21t12 C21t13

TABLE 3 Randomization of experiment [Cl−] (% w of Cl⁻/w of Random. total solution) C7 0.021 0.06 C10 0.075 0.09 C9 0.120 0.08 C12 0.141 0.20 C8 0.290 0.07 C6 0.305 0.05 C15 0.337 0.50 C17 0.439 1.50 C1 0.501 0.00 C18 0.525 2.00 C13 0.548 0.30 C14 0.551 0.40 C19 0.582 2.50 C16 0.584 1.00 C4 0.603 0.03 C2 0.671 0.01 C3 0.727 0.02 C5 0.730 0.04 C0 0.739 NCA C20 0.766 3.00 C11 0.819 0.10 C21 0.941 6.00

FIG. 13 shows the behaviour of the impedance and phase angle of a sensor with CA layer exposed to 0.50% w of Cl⁻/w of total solution, these measurement values being taken between coplanar electrodes. At minute 0 the CA was dry, and its phase angle shows its capacitive character (≈−90°). However, when the solution interacts with the CA, its angle phase changes until it reaches −5°. It means that CA is not a pure capacitor anymore, but also an electrical resistor.

On the other hand, the magnitude of the impedance shows that its impedance decreases from 10⁸ to 10³Ω in order of magnitude when the chloride solution reacts with the aluminate. These results are consistent with the results of the study disclosed in “Study of the dielectric properties in the NaNbO3-KNbO3-In2O3 system using AC impedance spectroscopy”, E. Atamanik and V. Thangadurai, 2009, Materials Research Bulletin 44 (4):931-936. In this study, the behaviour of capacitance and impedance of different ceramic materials are analysed. In the end, the dielectric permittivity is defined by the following equations:

$\begin{matrix} {ɛ = {ɛ^{\prime} + {j\; ɛ^{''}}}} & (2) \\ {ɛ^{\prime} = \frac{z^{''}}{2\pi \; f\; ɛ_{0}{SdZ}^{2}}} & (3) \\ {ɛ^{''} = \frac{z^{\prime}}{2\pi \; f\; ɛ_{0}{SdZ}^{2}}} & (4) \end{matrix}$

wherein

-   -   ε′ and ε″ are real and imaginary parts of the dielectric         permittivity;     -   Z, Z′ and Z″ are the magnitude, real and imaginary parts of         impedance;     -   S is the area exposed to CA of the electrodes,     -   d is the distance between the electrodes,     -   f is the frequency, and     -   ε₀ is the dielectric constant of the vacuum (8,8542×10⁻¹²         C²/Nm²).

As equations (3) and (4) demonstrate, dielectric permittivity is inversely related to impedance, which is coherent with the previous results.

In contrast, parallel plate capacitors (between facing electrodes) of the same chamber (same hole 12 filled with the CA layer) show different results illustrated by FIGS. 15 to 18.

FIG. 15 and FIG. 16 represent Bode diagrams of the impedance value |Z| and the phase angle of a first pair of facing electrodes separated by a CA layer exposed to 0.50% w of Cl⁻/w of total solution (first parallel capacitor). FIG. 17 and FIG. 18 represent the same diagrams for a second pair of facing electrodes separated by the same CA layer exposed to 0.50% w of Cl⁻/w of total solution (second parallel capacitor).

Even when both parallel capacitors have the same changes as the coplanar capacitor (Resistive behaviour for the coplanar capacitor and Capacitive behaviour for the parallel capacitors), at the end of the experiment, parallel capacitors do not reach a quasi-perfect resistive behaviour as aluminate in the coplanar plate capacitor does (see FIG. 16 and FIG. 18). In addition, the impedance magnitude changes over time in one of the capacitors while in the other the impedance reaches a steady state during the experiment (FIG. 15 and FIG. 17). This difference could be explained by differences in the diffusion process.

That is the reason, in this embodiment with impedance measurement, the impedance is preferably measured between coplanar electrodes.

Preliminary results show that impedance difference (ΔZ) between initial dried CA (Z₀) and CA exposed to solutions (0.5, 1.5, and 6.0% w of Cl⁻/w of total solution) reach a steady state after 15 kHz as illustrated by FIG. 19 for coplanar electrodes. ΔZ is calculated by following:

ΔZ=Z−Z ₀  (4)

In addition, there are some ranges where the response signal shows a considerable noise: 37.6-52 kHz and 65.1-80.9 kHz. These ranges must or should be avoided for measuring the impedances. Consequently, the measurements are advantageously made in the following ranges:

-   -   16 kHz<f<37.5 kHz     -   52 kHz<f<65 kHz     -   81 kHz<f<99 kHz

Additionally, regarding the time response, we can note that there is not a significant difference between the final impedance difference at 90 min (t13) and the first impedance difference at 2 min (t1) as illustrated by FIGS. 20 to 25. FIG. 20 represents the impedance difference ΔZ versus Frequency in the frequency range [16 kHz; 37.5 kHz] for C15t1, C15t13, C17t1, C17t13, C21t1, C21t13, and FIG. 21 represents the time average impedance difference value over time t1 to t13 for C15, C17 and C21. FIGS. 22-23 and FIGS. 24-25 represent the same diagrams for the frequency ranges [52 kHz; 65 kHz] and [81 kHz; 99 kHz], respectively.

These curves (FIGS. 20 to 25) show that the frequency ranges [16 kHz; 37.5 kHz], [52 kHz; 65 kHz] and [81 kHz; 99 kHz] are the most appropriate ones for the alternate current when measuring the impedance between coplanar electrodes. The chloride concentration can be assessed from the measured impedance between coplanar electrodes in these frequency ranges of the alternate current.

Some of the major advantages of the above described methods and systems are:

-   -   the sensor is chemically stable (alkalinity inside concrete),     -   the sensor can withstand temperatures and mechanical stresses,     -   the sensor does not need extra protection, since the housing can         isolate the inner material from corrosive environment,     -   the sensor can be placed anywhere, near a corner,     -   the measurements are not affected by the presence of electrical         fields,     -   its construction is cheap.

Of course, it may not be necessary that the chloride sensor 1 includes both facing electrodes and coplanar electrodes for a same chamber filled with calcium aluminate. If the method based on capacitance measurement is used, a sensor with only facing electrodes on both sides of the chamber is sufficient. If the method based on impedance measurement is used, a sensor with only facing electrodes on both sides of the chamber or coplanar electrodes on one side of the chamber is sufficient.

Although some embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the present invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications and substitutions without departing from the invention as set forth and defined by the following claims. 

1. A system for assessing chloride concentration at one predetermined area of a porous or composite material, comprising: a sensor embedded in the predetermined area, an analyzer connected to the sensor, and a processing module connected to the analyzer, wherein the sensor includes two facing or coplanar flat electrodes, an intermediate layer arranged between the electrodes, the intermediate layer being in contact with the material of the predetermined area and including calcium aluminates, wherein the analyzer is configured to apply an alternate current between the electrodes and output an impedance value or capacitance value of the intermediate layer, and wherein the processing module is configured to compute a chloride concentration assessment in the predetermined area based on the impedance value or capacitance value outputted by the analyzer.
 2. The system according to claim 1, wherein the electrodes are facing electrodes, and wherein the analyzer is configured to output a capacitance value and wherein, for computing the chloride concentration assessment in the predetermined area, the processing module is configured to compute a relative permittivity value of the intermediate layer between the electrodes from the capacitance value outputted by the analyzer and to compute the chloride concentration assessment in the predetermined area based on the computed relative permittivity value.
 3. The system according to claim 2, wherein the frequency of the alternate current is included in [100 Hz, 5 MHz].
 4. The system according to claim 1, wherein the analyzer is configured to measure an impedance value between the electrodes, by applying an alternate current between the electrodes, and the processing module is configured to compute the chloride concentration assessment in the predetermined area based on the measured impedance value.
 5. The system according to claim 4, wherein the electrodes are coplanar electrodes.
 6. The system according to claim 4, wherein the frequency of the alternate current is included in the following group of frequency ranges: [100 Hz, 100 kHz]; [16 kHz, 37.5 kHz]; [52 kHz, 65 kHz]; [81 kHz, 99 kHz].
 7. A method for assessing chloride concentration in a predetermined area of a porous or composite material, by using a sensor embedded in the predetermined area, the sensor including two facing or coplanar flat electrodes, an intermediate layer arranged between the two electrodes, the intermediate layer being in contact with the material of the predetermined area and including calcium aluminates, the method comprising: measuring a capacitance value or an impedance value of the intermediate layer by applying an alternate current between the electrodes; and computing a chloride concentration assessment in the predetermined area based on the measured impedance value or capacitance value.
 8. The method according to claim 7, wherein the electrodes are facing electrodes, and the measured value is a capacitance value of the intermediate layer between the electrodes, and wherein the chloride concentration assessment is computed by: computing a relative permittivity value of the intermediate layer between the electrodes, and computing the chloride concentration assessment in the predetermined area based on the computed relative permittivity value.
 9. The method according to claim 8, wherein the frequency of the alternate current is included in [100 Hz, 5 MHz].
 10. The method according to claim 7, wherein the measured value is an impedance value of the intermediate layer between the electrodes, and wherein the chloride concentration assessment is computed based on the measured impedance value.
 11. The method according to claim 10, wherein the electrodes are coplanar electrodes.
 12. The method according to claim 11, wherein the frequency of the alternate current is included in the following group of frequency ranges: [100 Hz, 100 kHz]; [16 kHz, 37.5 kHz]; [52 kHz, 65 kHz]; [81 kHz, 99 kHz].
 13. A chloride sensor to be embedded in a predetermined area of a porous or composite structure, comprising: a housing, at least two facing or coplanar flat electrodes within the housing, an intermediate layer arranged between the electrodes within the housing, the intermediate layer being in contact, via at least one hole in the housing, with the material of the predetermined area and including calcium aluminates, and pin connectors connected to the electrodes via conductive lines and arranged for connecting the electrodes to an external device.
 14. The chloride sensor according to claim 13, further including a plurality of pairs of electrodes offset with respect to one another along an axis of the sensor, and connected to a plurality of pin connectors, an intermediate layer being arranged between the electrodes of each pair of electrodes, and at least a hole being arranged in the housing at the proximity of each pair of electrodes and opening into the intermediate layer.
 15. The chloride sensor according to claim 13, wherein the calcium aluminates are selected from the group of: CA, C₃A, C₁₂A₇.
 16. The chloride sensor according to claim 13, wherein the material of the housing is fiber glass or Bakelite or ceramic or Teflon.
 17. The system according to claim 5, wherein the frequency of the alternate current is included in the following group of frequency ranges: [100 Hz, 100 kHz]; [16 kHz, 37.5 kHz]; [52 kHz, 65 kHz]; [81 kHz, 99 kHz].
 18. The chloride sensor according to claim 14, wherein the calcium aluminates are selected from the group of: CA, C₃A, C₁₂A₇.
 19. The chloride sensor according to claim 14, wherein the material of the housing is fiber glass or Bakelite or ceramic or Teflon.
 20. The chloride sensor according to claim 15, wherein the material of the housing is fiber glass or Bakelite or ceramic or Teflon. 